Hydrogen is utilized in a wide variety of industries ranging from aerospace to food production to oil and gas production and refining. Hydrogen is used in these industries as a propellant, an atmosphere, a carrier gas, a diluents gas, a fuel component for combustion reactions, a fuel for fuel cells, as well as a reducing agent in numerous chemical reactions and processes. In addition, hydrogen is being considered as an alternative fuel for power generation because it is renewable, abundant, efficient, and unlike other alternatives, produces zero emissions. While there is wide-spread consumption of hydrogen and great potential for even more, a disadvantage which inhibits further increases in hydrogen consumption is the absence of a hydrogen infrastructure to provide widespread generation, storage and distribution.
One way to overcome this difficulty is through the operation of hydrogen fueling stations. At hydrogen fueling stations, hydrogen generators, such as reformers or electrolyzers, are used to convert hydrocarbons to a hydrogen rich gas stream. Hydrocarbon-based fuels, such as natural gas, LPG, gasoline, and diesel, require conversion processes to be used as fuel sources for most fuel cells. Current art uses multi-step processes combining an initial conversion process with several clean-up processes The initial process is most often steam reforming (SR), autothermal reforming (ATR), catalytic partial oxidation (CPOX), or non-catalytic partial oxidation (POX), or combinations thereof. The clean-up processes are usually comprised of a combination of desulphurization, high temperature water-gas shift, low temperature water-gas shift, selective CO oxidation, selective CO methanation or combinations thereof. Alternative processes for recovering a purified hydrogen-rich reformate include the use of hydrogen selective membrane reactors and filters.
The gaseous hydrogen is then compressed and stored in stationary storage tanks at the hydrogen fueling stations to provide inventory to fuel internal combustion engines and fuel cell vehicles. The storage of gaseous hydrogen at hydrogen fueling stations is extremely expensive due to its low density. Large volumes of gaseous hydrogen are required to provide sufficient inventory which results in a large footprint for the storage. This large footprint is problematic as space is at a premium at a fueling station.
In addition to the issues surrounding the space necessary for the storage of gaseous hydrogen at a hydrogen fueling station, ensuring the complete filling of vehicles is another issue related to the operation of a hydrogen fueling station. Compression of the stored gas into the vehicle would require a prohibitively large compressor to achieve the fueling rates required. Pressure equalization is used to fuel the on-board storage tanks of vehicles. The pressure differential between the storage at the hydrogen fueling station and the vehicle is used to drive the fueling process. High pressure is required to achieve “full” fill density. This high pressure requires a corresponding inventory of “low” pressure gas. Specifically, for 1 kg of gas above 350 bar, over 5 kg of gas below 350 bar is needed. The result is “stranded” gas in the storage tanks. The stranded gas is the low pressure gas which was needed to have a volume of high pressure gas available for dispensing.
FIG. 1 shows a comparison of vehicle percentage full versus storage inventory from data from a demonstration hydrogen fueling station. As shown in FIG. 1, when the inventory of hydrogen in the storage tanks is less than 75% a vehicle will not receive a “full” fill. The vehicles are “full” when the inventory in the storage tank is greater than 75% “full.”
In addition to the issues involved in ensuring the complete filling of vehicles, the rate of the fill is another issue related to the operation of a hydrogen fueling station. Differential pressure is used to drive the gaseous hydrogen from storage tanks to the vehicle. Therefore, the rate at which a vehicle is filled depends on the storage pressure. A high pressure differential corresponds to a high flow rate and shorter fueling time. A low pressure differential corresponds to a low flow rate and a longer fueling time. The Department of Energy (DOE) has provided targets for the average fill rate. The current target for the average fill rate is 1 kg/min (2006). The future target for the average fill rate is 1.67 kg/min (2010).
The storage pressure is related to the inventory in the storage tanks. FIG. 2 shows a data comparison of fill rate versus storage inventory from two demonstration hydrogen fueling stations. As shown in FIG. 2, the average flow rate can be reduced when the storage is not full. When storage is less than 85% full the fill rate can drop below the 1 kg/min target.
The present invention addresses the need to reduce both the cost and size of hydrogen storage at hydrogen fueling stations. In addition, the present invention also addresses the need to provide a complete and fast fill.